Electrodeposition of metals



March 8, 1966 G. E. HELMKE 3,239,439

ELEGTRODEPOSITION OF METALS Filed July 9, 1962 A T TOP/V5 5 UnitedStates Patent 3,239,439 ELECTRODEPOSITION 0F METALS George E. Helmke,Millington, N.J., assignor to Bell Telephone Laboratories, Incorporated,New York, N.Y., a corporation of New York Filed July 9, 1962, Ser. No.208,538 6 Claims. (Cl. 20452) This invention relates to theelectrodeposition of copper, gold or silver onto molybdenum or tungsten.More particularly, the technique utilizes a sequential combination ofalternating current and direct current to initiate an adherent depositonto the base metal. This expedient in combination with a specificplating solution has been found to give extremely useful and unexpectedresults.

The electrodeposition of copper, gold or silver directly onto molybdenumor tungsten is useful for the manufacture of many electrical devices,notably electron tubes. Because of difficulty in obtaining a truly cleansurface on which to initiate the electrodeposition, a lack of adherenceto the base metal often occurs, and in many cases the deposit isblistered. In some procedures this difliculty is overcome by sinteringthe piecepart in a hydrogen atmosphere furnace at a temperature somewhatbelow the melting point of the deposited metal.

This invention is directed to a newly developed plating technique bywhich copper can be electrodeposited onto molybdenum directly with goodadherence, and one which eliminates the need for the sinteringoperation. By formulating the electrolyte with either a gold or a silversalt it is possible to deposit these metals onto molybdenum in the sameway as copper. Further, by using the same operating conditions adherentelectrodeposits can be made directly on tungsten.

These and other aspects of the invention may be more fully understoodfrom a consideration of the drawing in which:

The figure is a schematic representation of an apparatus useful inconducting the plating operation of this invention.

The figure shows a plating tank 1 containing electrolyte 2. Suspendedwithin the electrolyte is a workpiece 3 and an anode 4. The anode ispreferably an insoluble metal such as platinum. Supporting the workpiece3 and electrode 4 are conductive holders 5 and 6 which engage electricalbus bars 7 and 8. The circuit for providing the desired current isconnected as shown. The circuit consists of a DC. power supply 9adjustable by variable resistor 10 and an AC. source 11 varied byautotransformer 12. These current supplies may be alternatively selectedby double pole, double through switch 13. The D.C. power supply wasrated at 0 to 15 v., 0 to amps. The A0. source provided 0 to 24 v.current, 0 to amps.

Specific examplary procedures of this invention are presented in thefollowing detailed description.

A typical plating solution formulation for copper deposition is shown inExample 1. This technique relies in part on the cleaning and striking(or flash plating) of the base metal in a single solution, and thereduction of copper ions at the cathode at an extremely low cathodeefficiency and high hydrogen evolution voltage.

Example 1 Sodium hydroxide grams/liter 53 Cuprous cyanide do 1.5 Sodiumcyanide do 6.7 Workpiece current density (A.C.)

amperes/square inch 7 Cathode current density (D.C.) do 2.3

The solution (Example 1) contains a relatively high concentration ofhydroxide, 5%, and a relatively low con- 3,239,439 Patented Mar. 8, 1966centration of copper cyanide, 0.15%. The pretreatment phase of theplating operation is carried out through the use of the AC. powersupply. A conventional D.C. supply is used for the flash plating phase.The two supplies are connected to the electrodes through the doublepole, double throw switch. The power supplied to the bath thus can bereadily changed from one power source to another. The pretreatingoperation owes much of its effectiveness to the scrubbing action ofhydrogen evolved when the part is alternately made anodic and cathodic.Copper does not deposit at either electrode during this operationbecause of the changing polarity. This alternating current treatment iscarried out for /5 to 20 seconds, following which the double pole,double throw switch is used to connect the direct current power supplyto the plating electrodes.

With direct current applied across the electrolyte, the part to beplated is held steadily at a negative (cathodic) potential. Copper andhydrogen ions reduce at'the cathode but because of the formulation ofthe electrolyte, this deposition takes places with a high hydrogendeposition potential and a low cathode efliciency for copper. Thisefficiency has been measured at less than 2%. The low efficiency of thisstrike contributes to the good adherence which parts plated in this bathexhibit.

The AC pretreatment, D.C. flash plating process is used to depositamounts of the order of 1 milligram per square inch or less of copper.This deposit can then be effectively built up to the desired thicknessby conventional copper plating techniques. Such procedures are wellestablished in the art. For instance, see Modern Electroplating, editedby A. G. Gray, pages 98 to 114 and 194 to 225 (1953).

Example 2 Sodium hydroxide grams/liter 40 Silver cyanide ..do 2 Sodiumcyanide do 6 Workpiece current density (A.C.) amperes/inch l0 Cathodecurrent density (D.C.) do 3 Example 3 Sodium hydroxide grams/liter 15Potassium gold cyanide do 2 Sodium cyanide do 2 Workpiece currentdensity (A.C.) "ampere/inch l Cathode current density (D.C.) do 4Example 2 has been used to deposit silver directly onto molybdenum ortungsten and Example 3 to deposit gold directly onto molybdenum ortungsten by the same procedure presented in Example 1.

The foregoing embodiments are given by way of example. Limiteddeviations from the specific current density values and platingconditions can be tolerated while still providing the unexpected resultsto which this invention is directed. For instance, potassium salts maybe substituted for the sodium salts to achieve similar results. Otherdeviations and permissible operating ranges are set forth below.

Prior art cyanide plating solutions typically use bydroxide-to-metalsalt ratios of the order of a fraction to one. As previously indicated,this invention relies in part on a significantly higherhydroxide-to-metal salt ratio. For the purposes of this invention thisratio should lie in the range of 10:1 to approximately :1. This, interm-s of percentage composition of the aqueous electrolyte, isexpressed as 3 to 8% hydroxide: 0.05 to 0.3% metal salt. The remainingingredient, a soluble cyanide salt, such as an alkali metal cyanide, isincluded to facilitate and maintain the dissolution of the metal salt.This component is used in any amount appropriate for this purpose,generally 1 to 30% The cathode current density useful for the AC.preplating operation generally falls within the range of 1 to 15amperes/inch The cathode current density for the DC. plating operationis preferably 1 to 5 ampereslinch These .current.densities,.as comparedwith prior art'procedures, are quite high, which contributes in part 'tothe success of .thepresent invention.

Various other modifications and extensions of this in- .vention willbecome apparent to those skilled in the art. All such variations anddeviations which basically rely on the teachings through which thisinvention. has advanced theiart are properly considered within thespirit and scope of this invention.

Whatiis clairnedis:

1. Arprocess for electro-depositing a metal selected from the groupconsisting of copper, gold and silver onto a substrate selected from thegroup consisting of molybdenum and tungsten which comprises the stepsof: placing the-substrate in an electrolyte comprising the followingaqueous solution:

Percent AOH 3 to 8 MCN 0.05 to 0.3

where A is selected from the group consisting of K and Na and MCN is asolubilized cyanide salt selected from the groupconsisting of coppercyanide, silver cyanide and gold cyanide, passing an alternating currentthrough 3. The process of claim 1 wherein the MCN is AuCN' and themetalelectroplated is .gold.

4. The process of claim 1 wherein the; MCN isJAgCN and themetalelectroplated is silver;

5. The process of :claim l wherein the current density at the substrateduring the AC. pretreatment is maintained within th'e range: 1 to. 15amperes/inch and thecathode current density during the DC.electrodeposit is =maintained within the range lto 5 amperes/inch 6. Theprocess of claim-1 wherein the electrolyte con tainsa soluble cyanidesalt in anamount suflicient to facilitate dissolution of the MCN;

References. Cited by the Examiner UNITED STATES PATENTS 2,915,44412/1959 Meyer 204-341 6/1960 Gulick 204-34 JOHNH. MACK, PrimaryExaminer. MURRAY TILLMAN, Examiner.

1. A PROCESS FOR ELECTRO-DEPOSITING A METAL SELECTED FROM THE GROUPCONSISTING OF COPPER, GOLD AND SILVER ONTO A SUBSTRATE SELECTED FROM THEGROUP CONSISTING OF MOLYBDENUM AND TUNGSTEN WHICH COMPRISES THE STEPSOF; PLACING THE SUBSTRATE IN AN ELECTROLYTE COMPRISING THE FOLLOWINGAQUEOUS SOLUTION: